Hi,
I have the same problem with UE 5.2.1. A variable (structure variable made of String, Enumeration and Actor) is reset by default value in Child Blueprint. My best guest is the variable is reset when I recompiled the parent blueprint or when modified the variable itself (it is a structure or the enumeration included in the structure). So may be when I modified these variable, it recompiled all the blueprints (parents and childs) setting the values of variable to default. As I cannot really work around this bug, I set the variable on beginplay for each Child Blueprint and the values of the variable is stored in separated variables in each Child Blueprint.
Hope it can help.